1. Field of the Invention
The present invention relates to a process cartridge, an electrophotographic apparatus and image forming method.
2. Related Background Art
As a type of a color image forming apparatus (color electrophotographic apparatus) of an electrophotographic system, there is an electrophotographic apparatus with an intermediate transferring member (intermediate transferring belt or intermediate transferring drum).
As for that apparatus, operation scheme will be described with reference to FIG. 1.
In FIG. 1, reference numeral 1 denotes a photosensitive drum (drum-shaped electrophotographic photosensitive member) as a first image bearing member, which is rotatably driven at a predetermined rotation speed (process speed) in the direction of an arrow. In addition, the photosensitive drum 1 undergoes electrifying processing uniformly at a predetermined polarity and potential with an (primary) electrifying means 2 during the rotation process, then receives exposure light 3 by an unillustrated exposing means (for example, laser beams or LEDs). Thus, an electrostatic latent image is formed corresponding to the first color component image (for example, the yellow color component image) of the target full color image. Subsequently, the electrostatic latent image is developed with the toner (yellow toner) of first developing means (yellow color developing means 41) so that a toner image (yellow component image) is formed.
The intermediate transferring belt 5 is rotatably driven at a surface speed almost equal to that of the photosensitive drum (for example, 97 to 103% based on the rotation speed of the photosensitive drum) in the direction of an arrow.
While the above described first color toner image (yellow component image) formed on the photosensitive drum 1 passes through the contact part between the photosensitive drum 1 and the intermediate transferring belt 5, transfer (primary transfer) is carried out to the external circumference face of the intermediate transferring belt 5 from the photosensitive drum 1 by a primary transferring bias applied onto the intermediate transferring belt 5 via the primary transferring means 6 from the bias battery 30. The primary transferring bias is, for example, 100 to 3,500 V.
After transferring a toner image to the intermediate transferring belt 5, transfer residual toner is removed from the photosensitive drum 1 with photosensitive drum cleaning means 13 so as to get prepared for electrifying, exposing, developing, transferring steps of the next color component.
In the same way as in the first color toner image, the second to the forth color toner images are sequentially transferred and superimposed onto the intermediate transferring belt 5. In the primary transfer step of the first to the third color, the secondary transferring means (secondary transferring roller) 7 and the electric charge producing means 9 are apart from the surface of the intermediate transferring belt 5.
After the synthesized color toner image corresponding to the target color image is formed onto the intermediate transferring belt 5, the secondary transferring means 7 is brought into contact with the intermediate transferring belt 5, a transfer medium P is conveyed to the gap between the intermediate transferring belt 5 and the secondary transferring means 7 from a sheet feeding roller 11 at a predetermined timing and that toner image is transferred to the transfer medium P (secondary transfer).
In addition, the transfer medium P having the toner image transferred thereto is introduced into a fixing means 15 to undergo heat fixing.
After completing the image transfer onto the transfer medium P, the electric charge providing means 9 is brought into contact with the intermediate transferring belt 5. For the apparatus shown in FIG. 1, a roller is used as the electric charge providing means 9.
A voltage (for example, a direct voltage+an alternating voltage) of a reverse polarity to the surface potential of the photosensitive drum 1 is applied to the roller so that transfer residual toner on the intermediate transferring belt 5 is charged to a reverse polarity opposite to the polarity of the photosensitive drum 1. The transferring residual toner charged to the reverse polarity is electrostatically transferred onto the photosensitive drum 1 from the intermediate transferring belt 5 in the contact part in contact with the photosensitive drum 1 (contact part) and in the vicinity thereof. As a result, the intermediate transferring belt 5 is cleaned (electrostatic cleaning). The foregoing is the operation scheme of the electrophotographic apparatus using an intermediate transferring belt.
In recent years, the full color electrophotographic apparatus has started spreading rapidly, but a full color electrophotographic apparatus requires more disposals as compared with a conventional monochromatic electrophotographic apparatus, giving rise to a problem of a certain inferiority in maintenance performance.
In order to solve this problem, for example, Japanese Patent Application Laid-Open No. 9-292812 proposes such a trial that an electrophotographic photosensitive member and an intermediate transferring belt are combined together into one unit to reduce the number of disposals to improve the user""s jam handling performance or efficiency of replacement work of respective units. And already, process cartridges in which the electrophotographic photosensitive member and the intermediate transferring belt are combined together have been put on the market.
However, electrophotographic photosensitive members mounted on these process cartridges are belt-shaped (photosensitive belts), and therefore the size of a process cartridge itself gets larger and none can be said to be easy to replace. In addition, it is disadvantageous in reducing the size of the main body of an electrophotographic apparatus.
Therefore, the present inventors investigated a process cartridge integrally supporting an intermediate transferring belt and a photosensitive drum (an intermediate transferring belt-photosensitive drum integrated process cartridge) using a photosensitive drum.
However, in the intermediate transferring belt-photosensitive drum integral process cartridge, it was found that there was such a problem that the image density at the time of operation of the part where the photosensitive drum and the intermediate transferring belt were in contact with each other (contact part) when they were left standing was different from the image density of the other part (non-contact part) (hereinafter referred to as contact irregularity).
Moreover, it was found that the contact irregularity was apt to occur in the case of using a drum-shaped electrophotographic photosensitive member (photosensitive drum) more than in the case of using a belt-shaped electrophotographic photosensitive member (photosensitive belt).
The reasons therefore are deemed as follows.
Firstly, the following are deemed to be the reasons why the contact irregularity takes place.
Moisture in an intermediate transferring belt intensifies sensitivity in the contact part on an electrophotographic photosensitive member, giving rise to a sensitivity difference from the non-contact part, which causes dense longitudinal belts in the image to appear in a cycle corresponding to the peripheral length of the electrophotographic photosensitive member.
In addition, the following is deemed to be the reasons why the contact irregularity is apt to occur in the case of using a photosensitive drum more than in the case of using a photosensitive belt.
In the case of a photosensitive drum, the thickness of its supporting member (an aluminum cylinder is frequently used) must be made comparatively thick (for example, 0.5 to 3 mm) for maintaining its shape as a rigid material, and the electrophotographic photosensitive member cannot allow the moisture received from the intermediate transferring belt to escape through the supporting member of the electrophotographic photosensitive member.
To the contrary, the supporting member of the photosensitive belt is comparatively thin (polyester resin etc. having a thickness of 0.05 to 0.2 mm is frequently used), and hence can allow the moisture received from the intermediate transferring belt to escape through the supporting member without difficulty.
That is, if a photosensitive drum is used, it tends to be influenced directly by the moisture of the intermediate transferring belt, but if a photosensitive belt is used, the moisture is discharged to the air to a certain level through the thickness direction of the photosensitive belt, and therefore, the influence of the moisture in the contact part can be alleviated.
As described above, the present inventors have found that a photosensitive belt is more advantageous than a photosensitive drum from the contact irregularity viewpoint.
However,
(1) an intermediate transferring belt-photosensitive drum integrated process cartridge is more advantageous for miniaturization,
(2) but it is difficult to drive a photosensitive belt at a constant speed with an inexpensive system, and therefore in the case of using a photosensitive belt, drive irregularity is apt to appear as density irregularity (banding) of longitudinal lines in half tone images.
On the other hand, in the case of a photosensitive drum, it is comparatively easy to keep the surface speed stable, and therefore banding is hard to cause.
Therefore, the present inventors tried to solve the problem of the contact irregularity with an intermediate transferring belt-photosensitive drum integrated process cartridge.
An object of the present invention is to provide a process cartridge in which an intermediate transferring belt and an electrophotographic photosensitive member are integrally held together, which is more miniaturized, but does not bring about image defects such as banding or coarseness, and prevents the contact irregularity from occurring to provide uniform image density, an electrophotographic apparatus having the process cartridge and an image forming method using the electrophotographic apparatus.
The present invention provides a process cartridge detachably mountable on the main body of an electrophotographic apparatus, comprising
a photosensitive drum for bearing a toner image; and
an intermediate transferring belt having a contact part with the photosensitive drum;
which are integrally held together,
wherein the moisture amount of the intermediate transferring belt at 23xc2x0 C./50% RH is less than 1% by weight and
the sum of the surface roughness Ra of the photosensitive drum and the surface roughness Ra of the intermediate transferring belt is less than 0.8 xcexcm.
In addition, the present invention provides an electrophotographic apparatus comprising:
a photosensitive drum for bearing a toner image;
a charging means for charging the photosensitive drum;
an exposing means for forming an electrostatic latent image on the photosensitive drum charged with the charging means;
a developing means for developing with a toner the electrostatic latent image formed on the photosensitive drum with the exposing means and forming a toner image onto the photosensitive drum;
an intermediate transferring belt having a contact part with the photosensitive drum for secondarily transferring onto a transfer medium the toner image having been primarily transferred from the photosensitive drum; and
a primary transferring means for primarily transferring the toner image from the photosensitive drum to the intermediate transferring belt at the contact part; and
comprising a process cartridge which integrally supports at least the photosensitive drum and the intermediate transferring belt and is detachably mountable on the main body of the electrophotographic apparatus,
wherein the moisture amount of the intermediate transferring belt at 23xc2x0 C./50% RH is less than 1% by weight, and
the sum of the surface roughness Ra of the photosensitive drum and the surface roughness Ra of the intermediate transferring belt is less than 0.8 xcexcm.
Further, the present invention provides an image forming method comprising:
an electrifying step of charging a photosensitive drum;
an exposing step of forming an electrostatic latent image on a photosensitive drum charged in the charging step;
a developing step of developing the electrostatic latent image formed on the photosensitive drum in the exposing step with a toner to form a toner image on the photosensitive drum;
a primary transferring step of primarily transferring the toner image formed in the developing step with a primary transferring means from the photosensitive drum to the intermediate transferring belt having a contact part with the photosensitive drum, and
a secondary transferring step of secondarily transferring the toner image primarily transferred in the primary transferring step onto a transferring material, and
using an electrophotographic apparatus having a process cartridge which integrally holds at least the photosensitive drum and the intermediate transferring belt and is detachably mountable to the main body of the electrophotographic apparatus,
wherein the moisture amount of the intermediate transferring belt at 23xc2x0 C./50% RH is less than 1% by weight and
the sum of the surface roughness Ra of the photosensitive drum and the surface roughness Ra of the intermediate transferring belt is less than 0.8 xcexcm.